This Application claims the benefit of the earlier filing date of Japanese Patent No. 10-240731, filed Aug. 26, 1998, the entire contents of which are hereby incorporated herein by reference.
The present invention relates to a multicarrier type radio transmission apparatus for combining signals of a plurality of channels with a plurality of carrier frequencies into one signal to effect radio transmission.
This application is based on Japanese Patent Application No. 10-240731, filed Aug. 26, 1998, the content of which is incorporated herein by reference.
The individual amplification system was first put into practice as the amplification system in the multicarrier radio transmission apparatus. In the radio transmission apparatus of individual amplification system, amplifiers 10-1, 10-2, . . . , 10-n of a number equal to the number of carrier frequencies (the number of channels) used are provided as shown in FIG. 1 and transmission signals of the respective channels (signals from signal generators 12-1, 12-2, . . . , 12-n) are amplified by the respective amplifiers 10-1, 10-2, . . . , 10-n. Since the signals input to the respective amplifiers 10-1, 10-2, . . . , 10-n are signals each corresponding to one of the channels, there occurs no possibility that the signal of each channel will interfere with the signal of another channel. Thus, the amplifiers 10-1, 10-2, . . . , 10-n can be operated in a high-efficiency operating region. Further, it is advantageous in the heat radiation because the amplifiers 10-1, 10-2, . . . , 10-n are separately provided for n channels.
Output signals of the amplifiers 10-1, 10-2, . . . , 10-n are combined in power by a power combiner 18 and supplied to an antenna (not shown). In order to prevent signals reflected from the power combiner 18 from being fed back to the amplifiers 10-1, 10-2, . . . , 10-n and causing distortion in the amplified signals, it is necessary to insert isolators 16-1, 16-2, . . . , 16-n between the amplifiers 10-1, 10-2, . . . , 10-n and the power combiner 18 so as to maintain isolation between the channels. However, if the isolators 16-1, 16-2, . . . , 16-n are inserted, there occurs a problem that great loss occurs to produce a large amount of heat.
Further, in order to simplify the construction of the power combiner 18, it is necessary to supply the amplified signals to the power combiner 18 via band-pass filters 14-1, 14-2, . . . , 14-n having high channel selectivity. However, since the pass bands are fixed in the conventional filters, it is impossible to change the frequency of the carrier signal of each channel if the pass bands of the filters 14-1, 14-2, . . . , 14-n are set to correspond to the bands of signals of the respective channels output from the signal generators 12-1, 12-2, . . . , 12-n. In the actual transmission system, there is a request for changing the carrier frequency of each channel to a carrier frequency of another channel assigned to the system or there will be a request for changing the bandwidth of the carrier frequency in the future, but the individual amplification system cannot cope with the request. In order to cope with the request at least to some extent, the pass bands of the filters 14-1, 14-2, . . . , 14-n are set equal to each other and the carrier frequency band of all of the channels is set as the pass band. However, in this case, it is also necessary to insert the isolators in order to improve the channel selectivity. Therefore, the power combiner 18 becomes complicated in construction.
In order to solve the problem of the individual amplification system, a collective amplification system was developed. As shown in FIG. 2, in the collective amplification system, signals of carrier frequencies of the respective channels output from the transmission signal generators 12-1, 12-2, . . . , 12-n are first combined by a power combiner 22 and then collectively amplified by an amplifier 24. Thus, since the amplifier 24 is not provided in the succeeding stage of the amplifier 24, it becomes unnecessary to connect an isolator in the preceding stage of the power combiner 22 and a problem of loss and heat generation which occurs in the individual amplification system by the presence of the isolators will not occur. However, since a plurality of channel signals are simultaneously input to the amplifier 24, the linearity of the amplifier 24 becomes important in order to prevent inter-modulation distortion between the channel signals (generally, the high linearity operation and the high-efficiency operation conflict with each other), but in recent years, the high-efficiency operation of a linear amplifier can be attained by various technical improvements. In this respect, the advantage in efficiency of the collective amplification system is recognized.
However, in the collective amplification system, the operation efficiency of approx. 40% at maximum can be attained when the maximum permissible number of channels are received, but if there is an unused channel, the efficiency is lowered. This is because the amplifier 24 must be operated in a low-efficiency operating region (low input power portion) when the number of channels used is small since the input power to the amplifier 24 is changed according to the number of channels used. Further, in the collective amplification system, since heat generation is concentrated in one portion of the amplifier 24, it becomes necessary to take a large-scale heat radiation measure. Since the number of accommodated channels is determined by the maximum permissible number of channels of the amplifier 24 and the value of the maximum permissible power of the filter 26, there occurs a problem that it is difficult to increase the number of accommodated channels after designing of the system. Further, there occurs a problem that large permissible power becomes necessary as the specification of the filter 26 in order to deal with a large number of channels.
Accordingly, it is an object of the present invention to solve a problem of a conventional multicarrier radio transmission apparatus of individual amplification system.
Another object of the present invention is to provide a radio transmission apparatus which can prevent inter-modulation distortion between channels in the amplifier and reduce the adjacent channel leakage power to attain the high-efficiency operation of the amplifier higher than the efficiency of the conventional individual amplification system.
Still another object of the present invention is to provide a radio transmission apparatus which is excellent in heat radiation and can attain the high-efficiency operation irrespective of the utilization factor of the channels.
Another object of the present invention is to provide a radio transmission apparatus which is highly flexible with respect to an increase or decrease in the number of accommodated channels.
Another object of the present invention is to provide a radio transmission apparatus which can cope with a difference in the transmission rate with high flexibility.
A radio transmission apparatus according to the present invention performs radio transmission by use of a plurality of carrier frequencies and comprises signal processing systems each including a transmission signal generator for generating a signal of one carrier frequency, an amplifier for amplifying the signal generated from the transmission signal generator, and a variable band-pass filter for permitting only the signal of the one carrier frequency among the output signal of the amplifier to pass therethrough; and a combiner for combining signals output from the variable band-pass filters of the plurality of signal processing systems into one signal and using the signal as a transmission signal.
In the above radio transmission apparatus, since a signal generated from the transmission signal generator and passing through the amplifier and variable band-pass filter which constitute one signal processing system is a signal of one carrier frequency, inter-modulation distortion between the signals of respective carrier frequencies will not occur in the amplifier. Further, since only the signal of one carrier frequency is permitted to pass through the filter, the adjacent channel leakage power is suppressed so as to make it difficult to generate inter-modulation distortion of the signal between the channels. Therefore, the individual amplifiers can be operated with an efficiency higher than that of the conventional individual amplification system. Generally, the amplifier having no inter-modulation distortion is operated with a lowest efficiency. In the conventional individual amplification system, the amplifier can be operated with an efficiency higher than that of the amplifier having no inter-modulation distortion since it is required for the amplifier that the adjacent channel leakage power is limited to be lower than a predetermined level. According to the present invention, since the leakage power is suppressed by the filter, it is not required for the amplifier that the adjacent channel leakage power is limited to be lower than the predetermined level. As a result, the amplifier can be operated with an efficiency higher than that of the conventional individual amplification system. Further, since the amplifiers are separately disposed for the respective channels, the heat radiation characteristic is improved so as to make it unnecessary to provide a large-scale heat radiation structure. Since the filter can be constituted by a filter having a resistance to withstand the power of a signal of one channel, a superconductive filter, for example, can be used as the filter of the radio transmission apparatus.
The transmission signal generators of the plurality of signal processing systems have carrier frequencies different for the respective signal processing systems.
Therefore, interference due to inter-modulation of the signal of another carrier frequency can be prevented and it becomes possible to easily combine the powers of the signals of the respective channels by use of the combiner, thereby making it possible to save the power of the radio transmission apparatus.
Further, the center frequency of the variable band-pass filter can be made variable with the bandwidth of the pass band thereof kept constant or both of the center frequency and the bandwidth of the pass band can be made variable.
Therefore, it becomes possible to cope with data transmission from data transmission with relatively low rate as in the case of voice to high rate data transmission as in the case of moving picture by determining the bandwidth of the pass band of each variable band-pass filter according to the signal transmission rate.
Further, if a superconductive filter is used as the variable band-pass filter, a refrigerator for cooling the superconductive filters, power monitoring means for monitoring powers of the signal outputs from the amplifiers, temperature monitoring means for monitoring the temperatures of the superconductive filters, and control means for variably controlling the operation efficiency of the refrigerator based on the power monitoring result obtained by the power monitoring means and the filter temperature obtained by the temperature monitoring means are provided.
Thus, the refrigerator can be efficiently operated and power saving can be attained.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.